1. Field of the Invention
This present invention relates to new polyisocyanates blocked with a mixture of blocking agents, to a method for their production and to their use in one-component polyurethane stoving compositions which can be stoved at comparatively low temperatures of 120.degree.-140.degree. C. and which exhibit significantly reduced thermal yellowing.
2. Description of the Prior Art
Multilayer coating systems which have a clear, glossy coating and are based on blocked polyisocyanates and organic polyhydroxyl compounds, for example polyhydroxy polyacrylates, as the top coat, are becoming increasingly important due to their excellent industrial finishing properties, particularly for coating automobiles.
Polyisocyanates which are suitable for this special field of application must, in particular, (i) be capable of crosslinking at a maximum stoving temperature of 140.degree. C., (ii) exhibit on very slight thermal yellowing during the stoving operation, and preferably exhibit no thermal yellowing at all, (iii) have a viscosity in concentrated organic solution which is sufficiently low for "high solids" applications, and (iv) be based on inexpensive raw materials.
Polyisocyanates blocked with 1,2,4-triazole according to EP-B 0,004,571, which have been described, in particular, as crosslinking agents for powder coatings, are essentially unsuitable for use in solvent containing coating compositions, because their solutions in organic solvents have comparatively high viscosities and are often unstable due to the tendency of the blocked polyisocyanates to crystallize.
Surprisingly, it has now been found that the above-mentioned conditions can be fulfilled in an optimum manner if, instead of pure 1,2,4-triazole, a combination of this blocking agent with 3,5-dimethylpyrazole, and optionally other blocking agents known in the art, is used for blocking lacquer polyisocyanates.
Both 1,2,4-triazole and 3,5-dimethylpyrazole have been described as blocking agents for isocyanate groups, e.g, they are both disclosed in U.S. Pat. No. 3,248,398 as blocking agents for long chain aliphatic monoisocyanates. In addition, 3,5-dimethylpyrazole is described in addition to other pyrazoles as a blocking agent for organic polyisocyanates in EP-A-0,159,117. These blocked polyisocyanates are intended in particular for use as components of electrodeposition lacquers, which are generally recoated, so that the thermal stability of these primer coats is not a problem. This prior publication also contains no reference to the suitability of this blocking agent for the preparation of blocked polyisocyanates having a high thermal stability.